How many have you tried? Is it possible the GPIO UART lines have been damaged? I have a 2B here that has no TX output and there the same was seen on this EmonTx not updating emonPi.thread on a Pi3.
Another test you could do is to load a 100% stock Raspbian image and use your USB programmer, 3 link wires and putty to view the serial console in putty, you should be able to run commands direct from putty over the GPIO UART as intended on a stock Raspbian image. If you can see the boot log and reach a command prompt but cannot run a command, the RX line of the Pi has a physical fault.